X-rays attenuate while passing through an object, depending on the X-ray attenuation coefficient unique to the object, such as Compton scattering or photoelectric effect.
X-ray imaging is radiography using the transmission characteristics of X-rays, and renders an X-ray image of the internal structure of a field of view (FOV) of a subject, based on the amount of X-rays attenuated during transmission of the FOV. In this regard, an X-ray imaging device includes an X-ray source emitting X-rays to the FOV, an X-ray sensor detecting X-rays that have passed through the FOV, and an image processor rendering an X-ray image of the FOV based on the X-ray detection result of the X-ray sensor.
Recently, while X-ray imaging is being rapidly substituted by digital radiography (DR) using a digital sensor, owing to the development of semiconductors and information processing technologies, a variety of improvements has been undertaken for X-ray imaging technology.
For example, FIG. 1 is a typical X-ray panoramic image of teeth and body portions surrounding the teeth, used in the dental field, and FIG. 2 is a conceptual view schematically illustrating a typical method of obtaining an X-ray panoramic image.
An X-ray panoramic image is obtained by obtaining X-ray images of the track of the dental arch of a subject while moving the X-ray source 4 and the X-ray sensor 5 on both sides of an FOV 2, i.e. the dental arch portion of a subject, to face each other, combining the obtained X-ray images, and deploying and displaying the relationship of the arrangement of teeth and body portions surrounding the teeth in a single transmission image. Thus, the X-ray panoramic image is used as a standard image most familiar to dentists, since the relationship of the overall arrangement of teeth and body portions surrounding the teeth can be easily viewed.
In order to obtain the panoramic image, a rotary shaft 3 between the X-ray source 4 and the X-ray sensor 5 has a two-axis drive system, i.e. the rotary shaft 3 linearly moves to a predetermined range of length while rotating to a predetermined range of angle. With this configuration, a focal range defined at a point between the rotary shaft 3 and the X-ray sensor 5 is moved on the track of the dental arch for scanning, such that the detection result of X-rays of teeth and body portions surrounding the teeth according to the sections of the track of the dental arch are obtained.
However, the X-ray panoramic image has problems of the transmission image, such as low accuracy of information regarding lengths, teeth overlapping, or blurring caused by the cervical vertebrae.
In another example, FIG. 3 is an X-ray CT image of a typical head, X-ray CT being used in the dental and related surgical fields, and FIG. 4 is a conceptual view schematically illustrating a typical method of obtaining an X-ray CT image.
An X-ray CT image is obtained by obtaining X-ray images by rotating the X-ray source 4 and the X-ray sensor 5 disposed on both sides of the FOV 2, i.e. the head of a subject, to face each other, and reconstructing the result of X-ray image obtaining using a reconstruction algorithm. Thus, a 3D image of the FOV is displayed. It is therefore possible to accurately display not only a 3D image (corresponding to the right bottom part of FIG. 3) of the entire FOV but also topographic images (corresponding to the left top and bottom parts and the right top part of FIG. 3) according to positions and directions desired by a user. Accordingly, X-ray CT images are used by dentists in areas such as implant surgeries, where a high degree of precision is required.
In addition, in order to obtain X-ray CT image as described above, the rotary shaft 3 between the X-ray source 4 and the X-ray sensor 5 rotates in a range of angles, fixed to the rotary axis extending in the longitudinal direction of the FOV 2, which is referred to as single-axis drive. Consequently, detection results of X-rays are obtained in several directions of the entire FOV 2 having the shape of a cylinder, defined by the rotation of the tangent between the X-ray source 4 and the X-ray sensor 5 on both sides of the rotary shaft 3.
However, a typical X-ray CT image emits a relatively large amount of radiation to the FOV compared to an X-ray panoramic image, and requires an X-ray sensor having a larger area, in particular, a larger width, which is problematic.
More specifically, the X-ray sensor must detect X-rays that have passed through the entire area of the FOV in several directions of the FOV in order to obtain a typical X-ray CT image.
A case of obtaining an X-ray CT image in an FOV having a first width w1 (the maximum width of the FOV) and a first height t1 (the maximum height of the FOV) using X-rays in the form of a cone beam, which is typically used in the dental field, will be described by way of example. FIG. 5 illustrates the corresponding case.
As illustrated in the drawing, in the case of typical X-ray CT image obtaining, the rotary shaft 3 between the X-ray source 4 and the X-ray sensor 5 rotates on a single axis in a range of angles, fixed to the center axis extending in the longitudinal direction of the FOV 2. Here, the second width w2 of the X-ray sensor 5 must be equal to or greater than the first width w1, i.e. w2≥(d2/d1)×w1, in which the magnification ratio d2/d1 of the X-ray imaging device, defined as the ratio of the distance d1 between the X-ray source 4 and the center axis of the FOV 2 with respect to the distance d2 between the X-ray source 4 and the X-ray sensor 5, is reflected, and the second height t2 of the X-ray sensor 5 must be equal to or greater than the first height t1, i.e. t2≥(d2/d1)×t1, in which the magnification ratio d2/d1 of the X-ray imaging device is reflected. Under these conditions, X-rays that have passed through the entire area of the FOV 2 in several direction of the FOV 2 can be detected. Thus, the FOV 2 has the shape of a cylinder having the first height, in which the diameter of the cross-section perpendicular to the longitudinal direction is the first width.
For reference, depending on the purpose, a half beam or half scan method may be used, on the assumption that the rotary shaft between the X-ray source and the X-ray sensor in X-ray CT image obtaining is identical to the center axis extending in the longitudinal direction of the FOV. In the half beam or half scan method, the second width w2 of the X-ray sensor is reduced to w2≥(d2/d1)×w/2 using full width half maximum (FWHM) asymmetric X-rays that transmit one of the left FOV and the right FOV of the rotary shaft.
However, regardless of which method is used, the width of the CT image-obtaining X-ray sensor must be equal to or greater than the radius of a circle, the cross-section of which is perpendicular to the longitudinal direction of the FOV, in order to render a 3D image of the entire FOV. Thus, the width of the CT image-obtaining X-ray sensor is significantly larger than the width of the panoramic image-obtaining X-ray sensor.
In fact, when attempting to obtain an X-ray panoramic image and an X-ray CT image in the same FOV, the panoramic image-obtaining X-ray sensor has the shape of a slit, the width thereof ranging from 5 mm to 20 mm, whereas the CT image-obtaining X-ray sensor has the shape of a square or a similar shape, the width thereof being similar to the height thereof.
In addition, the price of a typical X-ray sensor significantly increases with the size. The high price of the X-ray CT device is unavoidable due to the X-ray sensor having a large width. In addition, as the area of the X-ray sensor increases, the amount of radiation emitted to a subject also increases, which is problematic.